The present invention generally relates to contactor circuits. More specifically, to sensing a power supply derived from the contactor""s holding coil and immunizing the contactor""s circuitry from the adverse effects of transient induction inherent within the contactor circuit.
There is a constant desire to increase speed, decrease energy consumption and decrease the amount of physical space required in electrical circuits. Circuit designers are continually redesigning circuits to be faster, smaller and more energy efficient.
One goal circuit designers frequently focus upon is reducing the amount of physical space required for a circuit. Various factors influencing the circuit""s design include: power requirements, electric noise, component temperature, timing parameters, etc. Designers frequently utilize a variety of techniques to achieve their design goals. One such circuit design technique is to derive a power supply from the holding coil of a contactor. However, due to the physical structure of the contactor, transient inductive forces generated through normal use of the contactor may adversely affect the performance of the contactor""s circuitry coupled to the derived power supply. For example, contactors or relays that utilize dc coils may have electronic circuits which are used to control the device. The contactor""s electromechanical coil has resistance and this resistance can be used as a dropping resistor to derive a power supply for the electronics. When the coil resistance is used for the power supply, the power supply is exposed to transient forces, dL/dt, generated by the movement of the contactor""s armature.
When an armature moves from an open to a closed position, the movement causes the inductance of the magnetic circuit to increase as the air gap between the magnet and the armature decreases. The dL/dt generates a back electromotive force (EMF) that causes the current in the coil to decrease. Once the armature seals with the magnet, the inductance becomes fixed and the dL/dt decreases to 0. The current in the coil recovers to its initial level. If the contactor control circuit""s power supply is derived from the contactor holding coil, the derived power supplied to the contactor control circuit will decrease, or dip, with the contactor""s holding coil current. The effect of the transient inductance generated by the derived power supply may turn off the controller circuit when the holding coil current of the contactor decreases, thus causing nuisance tripping.
It is now apparent that the effects of dL/dt cannot be avoided on this type of power supply. If the electronics of the contactor circuit are designed to be tolerant of a power supply decrease due to dL/dt, a power supply derived from the coil resistance can be utilized. To overcome the decreased coil current caused by the inductance created during the closing of the armature, larger filtering capacitors were utilized. However, these larger capacitors occupy valuable physical space. Another filtering technique implemented to combat the. transient inductive effects is to use electrolytic capacitors. However, electrolytic capacitors typically have a shorter life expectancy than the contactor circuit.
Prior to the present invention, a need existed to provide a power supply status circuit connected to a contactor circuit that monitors the power supply derived from the holding coil of the contactor. Also, a need existed for maintaining the output of the power supply status circuit while tolerating variations of the derived power supply caused by the inherent transient inductive effect of the contactor""s physical and electrical structure.
This invention is designed to resolve these and other problems.
A power supply status (PSS) circuit is capable of monitoring a derived source of power from a contactor circuit while maintaining the PSS circuit""s output. The PSS circuit is capable of tolerating variations in the derived power supply caused by the inherent, transient inductive effects of the contactor""s physical and electrical structure.
According to the present invention, a robust PSS circuit has been developed with specific useful features for utilizing power derived from the holding coil of a contactor. As a result, use of the derived power eliminates the need for a separate and additional power supply. Also, the PSS circuit reduces the physical space previously required for filtering components necessary to utilize the derived power supply. In addition, the PSS circuit allows the use of longer lasting filtering components.
The first embodiment of the present invention is directed to a method of immunizing a contactor circuit from the inductive effects of a derived power supply, including: sensing a voltage signal from a power supply, the voltage signal having a switching range comprising an upper voltage and a lower voltage; providing a drive output responsive to the upper voltage; holding the drive output at a constant level as the voltage signal remains above the lower voltage; and, disabling the drive output in response to the voltage signal dropping below the lower voltage.
According to a second embodiment, the invention is directed to a device for immunizing a contactor circuit from the inductive effects of a derived power supply. The device comprising the contactor circuit having an input that receives a voltage signal. The voltage signal having a switching range comprising an upper voltage and a lower voltage. The contactor circuit providing a drive output in response to the voltage signal. The drive output being held at a constant level as the voltage signal remains above the lower voltage; and the drive output being disabled in response to the voltage signal dropping below the lower voltage.